Tradescantia albiflora-leaves were used in developing a determination method for chlorophyll in living leaves using a microscopic spectro photometer (MPV, Leitz).The wavelength of the maximal absorption for chlorophyll a was found to be at 670 nm and for chlorophyll b at 652 nm. To calculate the reference values the intensity of the transmitted light at 750 nm was measured. The absorption at 750 nm results only from the chlorophyll free structure in the leaves. To correct optical errors the two-wavelength method was used. The values gained in arbitrary units were calibrated against data measured in the conventional way. The resulting calibration line shows a very high correlation coefficient where r(2)=0.997. It was proved that the calibration line was also correct for determinations with leaves from other plant species.Using this determination method the chlorophyll content of small areas on the living leaf blade of Phaseolus vulgaris was determined.As consequence of the thicker mesophyll accompanying the veins the chlorophyll concentration is 30% higher in this tissue. A lower chlorophyll concentration was observed in the thinner leaf tip and in the oldest regions at the leaf ground.At the leaf tip, the leaf ground and in the tissue along the veins an iron deficit causes less bleaching than in the areoles. In the same leaf regions the bleaching herbicide atrazine induces rapid bleaching if supplied through transpiration stream. The application of Atrazine on the leaf surface gives rise to the contrary effect.All these phenomena seem to be a result of a differing water supply by the xylem vessels.